In order to meet a growing demand for wireless data traffic after commercialization of the 4G communication system, efforts are underway to develop an improved 5G or pre-5G communication system. For this reason, the 5G communication system or the pre-5G communication system is referred to as a system after a 4G network (beyond 4G network) communication system or after a long term evolution (LTE) system (post LTE). In order to achieve a high data rate, the 5G communication system is being considered to be implemented in a microwave (mmWave) band (e.g., 60 gigahertz (60 GHz) band). In order to mitigate a path loss of the radio wave in the microwave band and to increase a propagation distance of the radio wave, in the 5G communication system, beamforming, massive multi-input multi-output (MIMO), full-dimensional MIMO (FD-MIMO), array antenna, analog beamforming, and large scale antenna technologies are being discussed. In addition, in order to improve the network of the system, in the 5G communication system, technologies such as an evolved small cell, an advanced small cell, a cloud radio access network (cloud RAN), an ultra-dense network, a device to device communication (D2D), a wireless backhaul, a moving network, cooperative communication, coordinated multi-points (CoMP), interference cancellation, and the like have been developed. In addition, in the 5G communication system, hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC) which are advanced coding modulation (ACM) schemes, and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) which are advanced connection technologies have been developed.
Meanwhile, the Internet has evolved into an Internet of Things (IoT) network in which information is exchanged and processed among distributed components such as objects in a human-centered connection network in which humans generate and consume information. Internet of Everything (IoE) technology, which combines IoT technology and big data processing technology through connection with cloud servers, is also emerging. In order to implement IoT, technology elements such as sensing technology, wired/wireless communication, network infrastructure, service interface technology, and security technology are required. Thus, in recent years, technologies such as sensor network, machine to machine (M2M), machine type communication (MTC), and the like for connection between objects have been studied. In an IoT environment, intelligent Internet technology (IT) services that collect and analyze data generated from connected objects to create new value in human life can be provided. IoT can be applied to fields such as smart homes, smart buildings, smart cities, smart cars or connected cars, smart grids, health care, smart home appliances, advanced medical services, etc., through convergence and fusion between existing information technology (IT) technology and various industries.
Accordingly, various attempts have been made to apply the 5G communication system to the IoT network. For example, 5G communication technologies such as a sensor network, M2M, MTC, and the like are implemented by techniques such as beamforming, MIMO, array antennas, and the like. The application of cloud RAN as the big data processing technology described above is also an example of the convergence of 5G technology and IoT technology.
Wireless communication systems are not limited to providing an initial voice-oriented service, and are evolving into broadband wireless communication systems that provide high-speed and high-quality packet data services, such as high speed packet access (HSPA), long term evolution (LTE) or evolved universal terrestrial radio access (E-UTRA) of 3rd generation partnership projection (3GPP), high rate packet data (HRPD) and ultra mobile broadband (UMB) of 3GPP2, and communication standards such as IEEE 802.16e.
An LTE system, which is a typical example of the broadband wireless communication system, adopts an orthogonal frequency division multiple access (OFDMA) scheme in a downlink and a single carrier frequency division multiple access (SC-FDMA) scheme in an uplink. In the above-mentioned multiple access scheme, time-frequency resources to transmit data or control information for each user are allocated and operated such that the time-frequency resources do not overlap each other, that is, orthogonality is established, thereby distinguishing the data or the control information for each user.
The LTE system adopts a hybrid automatic repeat request (HARQ) scheme in which a physical layer retransmits corresponding data when decoding fails in an initial transmission. In the HARQ scheme, when a receiver fails to correctly decode data, the receiver transmits information negative acknowledgment (NACK) indicating decoding failure to a transmitter, and the transmitter retransmits corresponding data in the physical layer. The receiver combines data retransmitted by the transmitter with data that has not been decoded previously, thereby improving data reception performance. In addition, when correctly decoding data, the receiver can transmit acknowledgment (ACK) indicating successful decoding to the transmitter so that the transmitter can transmit new data.
In addition, the LTE system adopts a method of allocating resources to user equipment (UE) according to a channel state in order to improve reception performance of a downlink. A base station (BS) transmits a channel state information-reference signal (CSI-RS) to the downlink to allocate resources according to the channel state of the UE. The UE measures channel quality information (CQI) based on the CSI-RS and transmits the CQI to the BS. The BS can allocate an optimal frequency resource to the UE based on the CQI.
The LTE and long term evolution-advanced (LTE-A) systems operating as described above may support low-cost/low-complexity UEs of which some functions are limited. The low-cost UEs are expected to be suitable for MTC or M2M services, which mainly provide services such as remote meter reading, crime prevention, logistics, etc. In addition, a low-cost MTC UE is expected to be a means of realizing cellular-based IoT.
The low-cost MTC UE needs to improve the coverage compared to an existing UE because the low-cost MTC UE can operate in shadow areas or undergrounds of buildings where person cannot reach although it has low mobility according to characteristics of MTC/M2M services or IoT services. Even if the UE is not located in the underground of the building or the shadow area as described above, when the number of antennas of the UE and functions of the UE are limited in order to satisfy requirements such as low-cost and low-complexity, the reception performance deteriorates, so that it may be necessary to improve the coverage to compensate for the deterioration in the reception performance.
Accordingly, there is a need to develop a new technology for a UE requiring coverage enhancement like the low-cost MTC UE.
As described above, existing third generation evolved mobile communication systems such as LTE, UMB, 802.16m, etc., are based on a multi-carrier multiple access scheme. In order to improve transmission efficiency, the third generation evolved mobile communication systems may employ multi-input multi-output (MIMO) and may use various techniques such as beam forming, adaptive modulation and coding (AMC), channel sensitive scheduling, and the like. The above-mentioned various techniques described above may improve the transmission efficiency to improve system capacity performance by concentrating transmission power transmitted from various antennas according to channel quality or the like or adjusting an amount of data to be transmitted, and selectively transmitting data having excellent channel quality to a user. Since most of these techniques operate based on channel state information between an evolved Node B (eNB) or a BS and a UE or a mobile station (MS), it is necessary to measure the channel state between the eNB and the UE. At this time, the above-mentioned CSI-RS is used. The above-mentioned eNB refers to a downlink transmission and uplink reception device located at a certain place, and a single eNB performs transmission and reception for a plurality of cells. In a single mobile communication system, a plurality of eNBs is geographically dispersed, and each eNB performs transmission and reception for a plurality of cells.
The existing third generation and fourth generation mobile communication systems such as LTE/LTE-A utilize MIMO technology for transmitting data using a plurality of transmission/reception antennas in order to increase the data rate and system capacity. The MIMO technology spatially separates and transmits a plurality of information streams by utilizing a plurality of transmission/reception antennas. In this manner, a method of spatially separating and transmitting the plurality of information streams is referred to as spatial multiplexing. In general, to how many information streams spatial multiplexing can be applied depends on the number of antennas of a transmitter and a receiver. Generally, to how many information streams spatial multiplexing can be applied is called a rank of corresponding transmission. The MIMO technology that is supported by standards up to LTE/LTE-A Release 11 supports spatial multiplexing for a case in which the number of transmission and reception antennas is respectively 8, and up to 8 ranks are supported.
Accordingly, there is a need to develop a new technique for a UE and an eNB which transmit and receive a plurality of reference signals in order to use the MIMO technique.